1. Field of the Invention
The present invention generally relates to an organic transistor, and especially relates to an organic electro luminescent (EL) transistor that is useful as a self-luminous organic EL display, and capable of switching and luminescence functions by combining an organic light emitting diode (LED) and an organic transistor.
2. Description of the Related Art
In recent years, research has actively been carried out about organic EL elements because it has become clear that the organic EL elements are superior to luminous devices that deploy inorganic materials for reasons such as (1) a lightweight structure can be provided, (2) a wide area display can be easily obtained, and (3) various luminescences can be obtained. An organic EL display using an organic EL element provides high luminosity with a thin shaped configuration and a fast response speed, and is therefore considered to be a promising next-generation display device that will replace liquid crystal displays that are presently used. Further, an organic semiconductor material is attracting attention as a material to replace amorphous silicon or poly silicon in TFTs (thin film transistors) that are used as display driving (controlling) devices.
Research of organic transistors has been actively conducted since the beginning of the 1980s, and fundamental characteristics of low molecular and high polymer organic compounds for semiconductor film have been studied. However, because an organic semiconductor material as above has lower electric charge transfer speed and higher resistance than an inorganic semiconductor material, the organic semiconductor material has seldom attracted attention for practical applications. Starting recently, research of organic semiconductor materials is being actively conducted for application to a wide area display element of the next generation, so that present liquid crystal displays, which are often used in portable electric devices, can be replaced with displays using organic material that features lightweight and flexibility. For example, TFT that is formed with a pentacene film on a highly doped silicon substrate, which realizes electric charge transfer speed of 0.52 cm2/V-sec, is proposed (JP,10-270712,A).
TFT formed with pentacene has shown problems in that vacuum membrane formation is required to form pentacene into a thin film, and that adhesion of pentacene to a substrate is weak. Then, C. J. Drury et al. proposed a totally-organic TFT that employed polyimide for the substrate, poly vinyl phenol (PVP) as an insulated material, poly thienylenevinylene (PTV) as a semiconductor material, and doped poly aniline as an electrode material which realizes an electric charge transfer speed of 3×10–4 cm2/v-sec (APL Vol. 73, No.1 (1998) 108).
However, the electric charge transfer speed attained by the TFT proposed by J. Drury et al. is still low, and improvements are desired. From these facts, it has become important that improvements be based on total technology. That is, in order to attain an electric charge transfer speed comparable to amorphous silicon, the technology of TFT structures and the technology of TFT production processes have to be added to the technology concerning organic semiconductor material.
As for organic semiconductor materials, for example, (1) low molecular compounds such as pentacene and metal phthalocyanine, (2) short chain oligomers such as C3–C8 n-thiophene, and (3) long chain polymers such as poly thiophene and poly phenylenevinylen are available. The long chain polymer is known as a π-conjugate system conductive polymer, wherein an electric charge moves along a low molecular compound, an oligomer, and a polymer by conjugation of atomic orbits between adjacent atoms that are multimeric. Depending on the conjugation status of orbits of adjacent molecules, electron transfer between the molecules becomes possible. An organic thin film of a low molecular compound or a short chain oligomer is known to show the highest degree of charge transfer among organic materials, and the low molecular compound or the short chain oligomer that shows such a high degree of charge transfer is made into an ordered arrangement, and formed into a thin film by vacuum evaporation. In the ordered arrangement of the thin film, atomic orbits overlap, which overlap is considered to facilitate the transfer of an electric charge between adjoining molecules. A long chain polymer that has great solubility provides a low cost film by spin-coating and dip-coating, which certainly is an advantage. However, a problem is that the electric charge transfer speed is low because the ordered arrangement is not available. Thus, an organic semiconductor material that has sufficiently high electric charge transfer speed has not been found.
As mentioned above, while an organic semiconductor material offers a possibility that TFT will be realizable with cheaper and easier membrane formation technology, such as vacuum evaporation, spin-coating, dip-coating, printing, and ink jetting, which is an advantage, there is a problem in that the electric charge transfer speed is lower than desired. Typical electric charge transfer speed is 0.001–0.1 cm2/V-sec in the small molecular compounds and the short chain oligomer, and 0.0001–0.01 cm2/V-sec in the long chain polymer. The highest electric charge transfer speed of an organic semiconductor material reported so far is 0.7 cm2/V-sec of a pentacene thin film.
Under the state of the art as described, even if TFT having an electric field effect transistor (FET) structure is simply employed as a driving unit of an organic EL element, the electric charge transfer speed is still low, and it is difficult to obtain a desired operation speed at desired power consumption.
Another approach is to use an electrostatic induction n-type transistor (SIT) as a switching element that provides comparatively favorable ON current value at a low electric charge transfer speed. SIT is a transistor in which electric current flows vertically through an active layer, while, generally, in TFTs, electric current flows horizontally through an active layer.
FIG. 11 is an outline sectional drawing of a SIT. A SIT generally includes a P+ gate 103 that is inserted in a semiconductor layer 104 that is sandwiched by an N+ source electrode 101 and an N+ drain electrode 102. When voltage is applied to the P+ gate electrode 103, a depletion layer 105 (two regions shown by dotted lines in FIG. 11) expands in the semiconductor layer 104. As the voltage is increased, the two regions grow and touch each other at a certain voltage, which is called the contacting voltage. When the gate voltage is smaller than the contacting voltage, SIT will be in an ON state. In order to turn the SIT to an OFF state, it is necessary to apply a negative voltage between the P+ gate 103 and the N+ source electrode 101 such that the potential level is raised. That is, the current IDS that flows between the N+ source electrode 101 and the N+ drain electrode 102 is a function of the height of a potential barrier produced by the voltage applied to the P+ gate 103 and drain voltage VD. A SIT that operates in this manner is called a normally-ON SIT, which features (1) providing high speed operation because no carrier is injected from the gate, (2) providing high ruggedness (large current can flow) because no current concentration is present, (3) being a voltage driven device, (4) showing an unsaturated current-voltage characteristic, and the like.
As for a SIT using an organic semiconductor, a vertical type TFT is reported (Kudo et al., T.IEE Japan, Vol.118-A, No.10, (1998) P1166 –1171), wherein copper phthalocyanine (CuPc) is sandwiched by a source electrode and a drain -electrode, and a gate electrode consists of slit-like aluminum formed by vacuum evaporation, and embedded in the copper phthalocyanine (CuPc) that is an organic material. All films of this TFT are formed by the vacuum evaporation method on a glass substrate. Further, in order to form a Schottky barrier near the interface of an organic molecule film of CuPc, which is formed by vacuum evaporation, and the slit-like aluminum gate electrode of the SIT, the slit-like aluminum is exposed to the atmosphere immediately after carrying out the vacuum evaporation so that the aluminum surface is oxidized. Problems with this SIT are that: since all the films are formed by the vacuum evaporation method, costs of production equipment becomes high, and, therefore, productivity becomes low; and because the process includes exposure to the atmosphere for oxidizing the metal, control is difficult, and reproducibility is not satisfactory.